This invention relates to a charging apparatus that charges a rechargeable battery at a constant voltage.
The lead storage battery, nickel cadmium battery, and other rechargeable batteries are well known for their ability to be charged repeatedly. In recent years, with the miniaturization and weight reduction of electronic equipment such as portable communication devices, notebook personal computers, and video cameras, the small but high capacity lithium-ion rechargeable battery has taken the limelight. As described in Japanese Patent Disclosure 5-111184, issued Apr. 30, 1993, the lithium-ion rechargeable battery is different from the constant current charged nickel cadmium battery in that it is charged with a constant voltage while limiting the charging current to a prescribed value.
FIG. 1 shows an embodiment of a prior art constant voltage charging circuit. This circuit charges a rechargeable battery B contained in a battery pack P. The battery pack P also contains a protection circuit S to prevent excessive charging or discharging of the rechargeable battery B.
This charging circuit is provided with a Direct Current (DC) power supply 1 for battery charging, a constant current control circuit 2 for controlling the charging current to the rechargeable battery B, and a constant voltage control circuit 3 for controlling tile charging voltage to keep the rechargeable battery B voltage from exceeding a prescribed value.
The constant current control circuit 2 is made up of two transistors TR1 and TR2, and two resistors R1 and R2. When the rechargeable battery B charging current increases, the voltage drop across R1 increases, increasing the base current in transistor TR2. Because of this, the emitter-collector resistance of TR2 drops, decreasing the base voltage of TR1. As a result, the emitter-collector resistance of TR1 increases, decreasing the charging current to the rechargeable battery B.
On the other hand, when the rechargeable battery B charging current decreases, the emitter-collector resistance of TR1 decreases, increasing the charging current to the battery B. In this fashion, the constant current control circuit 2 controls the rechargeable battery B charging current so as to be a fixed value (for example, 1C).
The constant voltage control circuit 3 is made up of a voltage detection circuit 4 connected between terminals A with the intention of measuring the voltage of the rechargeable battery B within the battery pack P, an operational amplifier (op-amp) S, a 4.2 V voltage reference device 6, and a diode 7. The op-amp 5 has its inverting input terminal connected to a voltage divider of the voltage detection circuit 4, its non-inverting input terminal connected to the voltage reference device 6, and its output terminal connected to the base of transistor TR1 through diode 7.
When the voltage measured between terminals A by the voltage detection circuit 4 is at or below the set voltage (4.2 V) of the voltage reference device 6, the output of the op-amp 5 is positive. Therefore, the diode 7 is reverse biased, and no current is pulled from the base of transistor TR1. Consequently, the constant current control circuit 2 charges the rechargeable battery without being affected by the constant voltage control circuit 3.
On the other hand, when tile voltage between terminals A reaches the set voltage of the voltage reference device 6, the output of the op-amp 5 goes to zero. This pulls current from the base of transistor TR1 into the op-amp 5, increasing the emitter-collector resistance of TR1, and reducing the charging current. In other words, charging is controlled such that the voltage between terminals A does not exceed the set voltage.
In this manner, under control of the constant current control circuit 2 and the constant voltage control circuit 3, the rechargeable battery B is charged by constant current and constant voltage as shown by the voltage-current (V-I) characteristics of FIG. 2.
In the above discussion, the rechargeable battery B voltage is taken to be the voltage measured by the voltage detection circuit 4 between terminals A. However, there is not only the rechargeable battery B between the terminals A-B, but there is also a resistive component due to the switching devices of the protection circuit S and connecting wires between A. The voltage drop due to this resistive component is included in the A terminal voltage. Consequently, the actual terminal voltage of the rechargeable battery B (namely, the voltage between terminals C-D in FIG. 1) is not constant but drops at high charging currents as shown in FIG. 3.
The resulting charging characteristics (namely, battery voltage V, charging current I, and charging capacity C) of the rechargeable battery B are shown by the solid lines of FIG. 4. Consequently, the actual battery charging time is longer than that expected by the charging characteristics (broken lines) corresponding to the V-I characteristics of FIG. 2.
It is thus the object of the present invention to provide a charging apparatus with a compensation circuit which reduces rechargeable battery charging time by compensating for the voltage drop due to the resistive component.